This invention relates to earth boring tools, and more particularly to drills for boring through rock. More specifically, the invention relates to rock drill bits especially adapted to deep hole drilling.
There is an increase in world demand for mineral sources of energy, particularly oil and gas. Economic and political considerations have made it expedient, as well as profitable, to explore much deeper formations not only for these hydrocarbon sources of energy but also for geothermal sources of energy. While world needs today are basically being supplied with wells which are at the most 10,000-12,000 feet deep, it has now become necessary to drill for these commodities to depths which may reach in excess of 30,000 feet. Accordingly, a "deep hole" as the term is referred to in the present specification will be understood as one which is greater than about 12,000 feet deep.
The physical properties of the shallower wells currently being exploited are quite different from those experienced when the hole is over 12,000 feet. Temperature increases and the compressive strength of rocks increases as the hole depth deepens and this presents problems at depths above 12,000 feet and particularly difficult problems at depths of over 18,000 feet. Generally the drilling speed is a deep hole decreases significantly with increase in depth. The pressure and compressive strengths prevailing at such depths causes rolling impact type drilling tools such as rotary one drill bits to lose their effectiveness, and the bit teeth begin to "track" in their own pounded impression at the bottom of the hole. The bit teeth no longer fracture and gouge the rock. In deep holes, the rock has been measured as having compressive strengths of 100,000-180,000 psi and requires shearing or shaving forces to effect drilling, rather than roller cone drill bits which operate by compressive fracture means.
Furthermore, in rock formations at lesser depths conventional rotary cone drill bits will often cut oversize because of soft rock formations and the blast nozzles action of the mud flow. However, as the depth increases in hard rock the borehole tends to close behind the drill because of the earth overburden pressure and causes drill withdrawal problems.
Another serious problem encountered with the rotary type drill bits in attempting to drill a "deep hole" involves the load of the drill string on the bit. In drilling a "deep hole" it is common to bore the first portion up to about 100 feet in depth with an earth auger at a diameter of 20-24 inches. A liner is then put in the bore to this point and cemented between the liner and the earth by conventional means. Thereafter a 17-inch diameter three roller cone type bit may be used to penetrate clay and soft sand. This type of drilling will proceed for approximately another 1,000 feet depending on the nature of the rock whereupon the bore is again lined with a suitable steel liner and cemented in. The bit diameter may then be reduced to approximately 13 inches for the next 4,000 feet. As the depth increases, the diameter of the bit is continually stepped down, for example, to a 61/2 inch diameter bit for the ultimate depths, e.g., up to 25,000 feet. Reduction in the size of the bit necessitates a reduction in the size of the bearings in the roller cones supporting the weight of the drill string above. At such extreme depths, the load of the drill string can reach and exceed two million pounds. Although counter-balanced, impact often puts the full load of the drill string on the bearings. While the bearings should be getting larger to withstand the increasing loads, they are, of necessity, made smaller. In addition, in geothermal wells, corrosive chemicals usually found in such wells rapidly destroy rotary cutting bearing materials.
In addition, roller cone type bits and diamond bits require a shock absorber accessory when drilling. Such an accessory is necessary as the rock gets deeper and harder because of the chattering of the bit when fracturing the rock.
A further problem encountered with known drill bits is that caused by the difficulty in fracturing hard rock close to or on the axis of the drill bit. Because of this, some bits have been provided with core breakers. However, the core breakers wear and blunt thereby providing problems in deep hole drilling.
As indicated above, the plastic nature of the hard rock formation at such deep hole depths results in a closing in of the sidewalls of the bore. A decrease in diameter of as little as 1/32 of an inch in deep holes may be sufficient to prevent return of the drill to the surface. The close-in occurs in a very short time, for example, starting in seconds and continuing to within about 3 minutes of passing through a given level. Thereafter, the flow of the rock inward closing the bore holes usually stabilizes. Accordingly, it is essential to ream immediately after drilling.
Prior structures have involved the positioning of reamers and non-cutting drill stabilizers along the drill string at spaced levels to allow the stabilization of the formation between the drill bit and to maintain the diameter of the hole to permit withdrawal of the drill bit as this becomes necessary.
Another problem encountered in deep hole drilling is the thickening of the mud fluid because of loading with rock cuttings and suspension solids. Drilling muds usually contain some dispersed solids initially, e.g., bentonite clay, to aid in the dispersion of rock cuttings from the hole bottom and additional suspension solids are added as the drilling deepens the hole. The nature of such cuttings is important to the ability of the drilling mud or slurry to remove such cuttings efficiently. With conventional tri-cone bits, cuttings may be too large to be suspended and carried up outside the drill pipe and mud weights may reach 17 pounds per gallon, rather than a preferred 9 to 11 pounds per gallon. This requires high energy pumps and greatly increased pumping pressures to lift the solids from the bottom of the hole.
Another problem experienced with prior art deep hole drilling structures has been the tendency of such drill bits to deviate from a true line, particularly upon encountering an off center force such as a geologic fault or tilted geological formation. As the drill enters such a fault or tilt, the resistance to descent on one side of the drill head is increased on the one side over what it is on the opposite side; and accordingly, the direction of the drill head is easily diverted, usually to head into the tilted rock face at 90 degrees to the face. Rotary cutter type drill bits are particularly subject to wandering from the predetermined bore line.
Another problem in drilling relates to field servicing of the drill head. It is common practice for a driller to estimate the number of drill bits which will be required for a given bore and to order a supply for replacement. Once a drill bit has been removed from the end of the string, it is usually returned to the manufacturer for servicing or reconditioning at a remote site instead of at the drilling site.
Prior bits for deep holes have also included diamond studded drills wherein diamonds are embedded in a suitable metal matrix to form an abrasive or grinding surface for cutting through rock of the type one experiences at great depths as in a deep hole. Difficulties are experienced with such diamond bits when the weight of the drill string becomes excessive. At such heavy loads, the diamonds fracture and when this happens, the drill head soon loses its effectiveness. Such diamond studded drill bits are extremely expensive.
The present invention represents an improvement on prior structures. There are no bearings in the improved structures of the present invention, and consequently the improved drill bit is better able to withstand the excessive loads imposed by both drill string and the drill mud at extreme depths. In addition, cutters including end cutters for shaving the bottom face of the hole are mounted in a secure manner without the use of screws or welding.
The problem of "rock creep close-in" can be overcome with the structures of the present invention since a suitable length of cutters functioning as reamers is provided to maintain the diameter of the bore for a sufficient period of time after the drill head has passed a given bore depth to overcome any tendency of the drill bit to become locked in against removal. Because of the structure of the devices of the present invention, there is less tendency for the bit to wander from a predetermined bore path and specific designs are readily provided when wandering is a serious problem as in the case of tilted geological formations. Still further, the drill bits of the present invention are easily serviced in the field. In geothermal exploration, the improved bits hereof are not as susceptible to damage by corrosive materials. The structure of the rock cutting elements not only makes them readily replaceable, but enables the cutters to be adjusted to accommodate variations in the nature of any rock encountered by the drill bit. The cutters may also be adjusted in the field to account for wear on the cutters.
The front rake angles of the cutters are critical for chip formation, cutters with proper rake angles can easily be installed. Negative front rake angles normally produce fine powdery grain chips of 50 to 200 micron size. Zero front rake angles produce small rock chips like flakes. Positive rake angles up to 12 degrees inward from center will, for example, produce chips up to 11/2" diameter by 1/8" thick in an 8" diameter drill bit.
Still further, the structures of the present invention enable improvement in the specifications for the drilling mud whereby the overall column weight may be adjusted favorably for suitable removal of the products of drilling.
An advantage over the prior art structures afforded by the present invention is that the torque for cutting through rock is, at deep levels, reduced over that which is required for diamond drills and rotary cutting drills. Accordingly, power requirements are reduced. Still further, the structures of the present invention are adjustable to maintenance of the diameter of the bore even though the cutter elements experience wear in the course of boring.
Vibration, which loosens bolts and threaded locking screws for holding the cutter elements in the historical bit utilizing such elements, does not affect the structures of the present invention which depend on a wedge system.
A further advantage of a bit in accordance with the present invention is the facility with which bits may be repaired, serviced, or changed on the drilling site, including bit changes necessitated by a change in the geological formation being drilled or merely because of the depth of the hole.
The drill bit of the present invention enables higher drilling speeds, particularly in deep holes, including holes of over 18,000 feet. Moreover, the advantages of on-site service and maintenance of the bit and higher drill speeds render the bit useful in drilling at less than 12,000 feet.